Prostacyclin is a vasodilator and inhibitor of platelet aggregation produced by the endothelium. Because of these properties, several prostacyclin analogues are being evaluated in patients with peripheral arterial occlusive disease. the safety and efficacy of a novel, long-acting prostacyclin analogue, UT-15, were studies in patients with severe intermittent claudication. Methods: Seven patients with stable intermittent claudication considered Fontaine classes IIb-III were admitted to the hospital for intravenous infusion of UT-15. A symptom-limited dose-escalation protocol beginning with placebo and then increasing doses of drug (60 minutes at each dose step) was begun, followed by a 2 hour maintenance phase at the maximum well-tolerated infusion rate. Lower limb blook flow hemodynamic measurements were assessed serially with ultrasound, segmental arterial pressures and pulse volume-recording amplitudes. Results: Common side effects at peak dose included headache and nausea. No serious adverse events occurred during hospitalization. The optimum infusion rate was 10 ng/kg/min in most patients. The mean (+-SEM) lower limb blook flow velocity, of all patients combined, increased by 33+110% (p=0.016) without significant change in blood pressure or heart rate. Arterial blood flow in the ankle became apparent during maintenance infusion in 2 of 4 patients without ultrasound detectable blood flow at baseline.